The present invention relates to a mixing system for an exhaust gases after-treatment arrangement, for example in an exhaust gas pipe. Said system is especially designed to improve the mixing of a fluid with the exhaust gases of a thermal engine, while also preventing the solid deposits of said fluid on the pipe wall. The present invention can be used for example in an exhaust pipe of a diesel engine wherein an aqueous solution of urea is injected in view of an after-treatment of the exhaust gases.
Exhaust gases formed in the combustion of fuel in an internal combustion engine may contain a proportion of undesirable substances such as nitrogen oxides (NOx), carbon monoxide (CO), un-burnt hydrocarbons (HC), soot, etc. . .
To reduce air pollution, vehicles are therefore equipped with various after-treatment systems that deal with undesirable substances in exhaust gases.
A common exhaust gases after-treatment system is a so called selective catalytic reduction (SCR) system. Exhaust gases wherein ammonia is added as a reducer is treated in a specific catalytic converter where nitrogen oxides are converted into water and nitrogen which are both non toxic substances. Ammonia is introduced in the form of urea in an aqueous solution from which ammonia is obtained through hydrolysis. Urea is usually nebulised in the exhaust gas upstream of the catalytic converter. To this end, a urea injection nozzle is fitted on the exhaust line upstream from the catalytic converter.
A problem with this type of exhaust gases treatment is that, before it has transformed into ammonia, urea can crystallize. In concrete terms, the aqueous solution of urea which is sprayed through the nozzle inside the exhaust pipe, according to a direction which is angled with respect to the exhaust gases flow direction, tends to form a solid deposit on the exhaust pipe wall, on the internal side thereof, for example opposite of the injection point. The consequence is that the cross section of the exhaust pipe is progressively reduced, which makes the engine efficiency decrease and which can seriously impair the engine operation in the long term.
Many prior art devices are not fully effective since they do not make it possible to achieve the complete chemical decomposition of liquid urea into gases and/or a satisfactory mixing of urea with exhaust gases.
One conventional device, generally referred to as a “swirl box”, makes it possible to achieve both above mentioned results to some extent. However, such a swirl box has several drawbacks. First of all, it needs to be long enough to allow the substantially complete chemical decomposition of urea and therefore it may be quite bulky. Moreover, when it has to be installed, it generally requires design adjustments of the surrounding parts. Besides, such a swirl box provokes backpressure and is quite expensive. Anyway, known swirl box designs do not always prevent effectively solid deposits.
It therefore appears that there is room for improvement in the systems for injecting a fluid in a pipe carrying exhaust gases and for mixing them.
It is desirable to provide an improved mixing system which can overcome the drawbacks encountered in conventional mixing systems, and particularly which prevents or at least limits the injected fluid from forming a deposit onto the pipe surface while also promoting a satisfactory mixing between said injected fluid and the exhaust gases.
For this purpose, the invention concerns, according to an aspect thereof, a mixing system for an exhaust gases after-treatment arrangement, said mixing system comprising:                a pipe having a longitudinal axis, in which exhaust gases can flow in a flow direction (FD);        a nozzle designed to inject a fluid inside the pipe from an injection inlet arranged in the pipe wall, according to an injection direction (ID);        a first mixing device positioned inside the pipe upstream from the injection inlet;        
wherein the first mixing device includes a peripheral portion comprising blades capable of creating a peripheral swirl along the pipe wall, and a central portion designed to create substantially no turbulence or a turbulence which is negligible compared to the turbulence created by the peripheral portion, and wherein the mixing system further comprises a second mixing device positioned inside the pipe downstream from the injection inlet, said second mixing device including a central portion comprising blades capable of creating a swirl inside the pipe.
By creating a peripheral swirl, the first mixing device, which is located upstream from the injection inlet, prevents the fluid from wetting the pipe wall, in particular but not exclusively opposite the injection inlet, or at least greatly reduces this wetting effect. As a result, solid deposits are avoided or highly limited.
The first mixing device is designed to generate turbulence mostly in the peripheral part of the pipe inner volume. For example, immediately downstream from the first mixing device, the turbulent kinetic energy of the fluid flowing in the pipe is at least ten times higher in the peripheral part than in the central part of the pipe inner volume. Another advantage of having substantially no turbulence created by the central portion of the first mixing device is that it limits backpressure. Indeed, the creation of a peripheral swirl is sufficient to achieve the aim of said first mixing device, i.e. avoiding deposits on the pipe inside wall.
Furthermore, the second mixing device, which is located downstream from the injection inlet, has a double function. Indeed, it creates a central swirl in the pipe, which complements the swirl created by the first mixing device, and furthermore helps breaking the fluid drops. As a result, the second mixing device promotes the mixing between the fluid (or the gases obtained by the decomposition of said fluid) and the exhaust gases and, in case the fluid is an aqueous solution of urea, improves the decomposition of liquid urea into gases.
With this arrangement, the mixing system according to the invention is much more effective than prior art systems in terms of evaporation, decomposition and mixing, and makes it possible to greatly reduce the solid deposits on the pipe inside surface.
Advantageously, the central portion of the first mixing device is substantially devoid of blades. Preferably, said central portion is devoid of any element, except possible stiffening means which generate substantially no turbulence.
In an implementation of the invention, the peripheral portion of the first mixing device forms a ring having a width, measured along a radial direction, which represents between around 30% and around 50% of the first mixing device radius.
According to an embodiment of the invention, the first mixing device comprises:                a substantially cylindrical sleeve having an axis and substantially forming a border between the peripheral portion and the central portion of said first mixing device;        a plurality of spoke members extending from the area surrounding the sleeve axis, and beyond said sleeve, the ends of the spoke members being in contact with the pipe wall so that, when the first mixing device is positioned inside the pipe, the sleeve axis substantially coincides with the pipe axis.        
Preferably, the peripheral portion of the first mixing device can comprise an outer ring of substantially identical outer blades capable of creating a peripheral swirl and an inner ring of substantially identical inner blades capable of deflecting the exhaust gases outwardly towards the outer ring of blades. The inner blades thus have a centrifugal effect and also contributes to the creation of the peripheral swirl. Providing two sets of blades also makes it possible to generate more turbulence, which enhances the mixing between the fluid and the exhaust gases. The inner blades preferably have a shape different from the shape of the outer blades.
For example, each outer blade extends from a downstream radial edge of a spoke member towards the downstream direction, said outer blade being further inclined towards the adjacent spoke member, and all outer blades being inclined similarly.
Each inner blade can extend from a downstream edge of the sleeve towards the downstream direction, said inner blade being further inclined outwardly. Thus, the inner blades all together form a kind of a cone frustrum which diverges towards the downstream direction. Preferably, each inner blade is further obliquely tapered from the sleeve towards its free end and therefore arranged to create a swirl in the same rotating direction than the outer blades.
As regards the second mixing device, it includes a peripheral portion which is preferably substantially devoid of blades. Thus, the pressure loss is limited. However, in said peripheral portion, the second mixing device can be provided with means designed to allow the positioning of said device inside the pipe.
According to a preferred implementation of an aspect of the invention, the blades of the peripheral portion of the first mixing device and the blades of the central portion of the second mixing device are oriented oppositely. By creating two opposite swirls, this arrangement improves the mixing of the fluid and gases inside the pipe and the homogenization of said mixture.
The second mixing device can comprise:                a substantially cylindrical sleeve having an axis and substantially forming a border between the peripheral portion and the central portion of said second mixing device;        a plurality of spoke members extending from the area surrounding the sleeve axis beyond said sleeve, the ends of the spoke members being in contact with the pipe wall so that, when the first mixing device is positioned inside the pipe, the sleeve axis substantially coincides with the pipe axis.        
The central portion of the second mixing device preferably comprises an outer ring of substantially identical outer blades and an inner ring of substantially identical inner blades. By providing two sets of different blades, the turbulence obtained is greater.
For example, each inner blade extends from a downstream radial edge of a spoke member towards the downstream direction, said inner blade being further inclined towards the adjacent spoke member, and all inner blades being inclined similarly.
Each outer blade can extend from a downstream edge of the sleeve towards the downstream direction, said outer blade being further inclined outwardly. Preferably, each outer blade is further obliquely tapered from the sleeve towards its free end and therefore arranged to create a swirl in the same rotating direction than the inner blades.
It can be envisaged that the central portions of the first and second mixers substantially have the same radiuses. Therefore, when seen along the pipe axis, the successive first and second mixing devices seem superimposed and look like a single structure having blades substantially on its entire cross section.
A specific application of the invention is the treatment of NOx in exhaust gases. In that case, said pipe is an exhaust pipe of a diesel engine and said second fluid is an aqueous solution of urea.
The invention makes it possible to obtain a satisfactory mixing between exhaust gases and urea and then, further downstream, between NOx and ammonia when urea has broken down. Therefore, it is possible to effectively reduce the NOx compounds and to achieve considerably lower NOx emissions. At the same time, the invention effectively prevents urea that has not broken down into ammonia yet from making a deposit on the pipe, in particular opposite its injection pipe, thereby increasing the service life of said exhaust pipe.
These and other features and advantages will become apparent upon reading the following description in view of the drawing attached hereto representing, as non-limiting examples, embodiments of a vehicle according to the invention.